Support is requested for a 2-year collaborative grant between scientists at the New York State Psychiatric Institute and Vanderbilt University to investigate the pathophysiology underlying OCD. The proposal bridges basic and clinical OCD research by integrating findings from the research team's ongoing human genetic studies into the proposed mouse experiments. The research plan thus capitalizes on the expertise of the team in 1) human OCD genetic studies, 2) development of transgenic mice, 3) biochemical assays, and 4) mouse behavioral analysis. Current understanding of the molecular and cellular abnormalities underlying OCD is limited, in part because post-mortem studies in humans have not been performed. In addition, mouse studies have not yet been convincingly linked to the clinical phenotype and genetic abnormalities seen in OCD patients. To date, the only gene which has been consistently linked to OCD in human genetic studies is SLC1A1, which codes for a protein that transports the neurotransmitter glutamate. In addition, there is evidence from human studies that abnormal regulation of glutamate transmission in striatum is correlated with OCD symptoms. This has led to the hypothesis that abnormal levels of the SLC1A1 glutamate transporter in striatum lead to 1) abnormalities in the glutamate system, 2) changes in brain structure, and 3) OCD symptoms. The proposed R21 will test this hypothesis using novel knock-in mouse technology. In the first aim, the researchers will use an efficient system they have previously developed for manipulating gene expression in mice called the FAST system (Flexible Accelerated STOP TetO-knockin). This will allow them to develop a novel knock-in mouse line called tetO-Slc1a1, which will permit precise regulation of SLC1A1 expression levels in brain regions implicated in OCD. They will then use this mouse line to generate abnormally high levels of the SLC1A1 glutamate transporter specifically in striatum. This will simulate the effect of the version of the gene found most commonly in OCD patients. In the second aim, the mice with abnormally high levels of Slc1a1 in striatum will be characterized by: 1) measuring glutamate system functioning~ 2) examining brain structure~ and 3) testing behavior in OCD-relevant paradigms that measure anxiety and repetitive behaviors. This will provide the first direct test of whether OCD-related dysfunction is caused by abnormal expression of the leading human OCD candidate gene. Completion of this grant will lead to an amenable system for 1) further dissection of the molecular, cellular, and electrophysiologic underpinnings of observed changes~ and 2) determination of whether there is a particular time in development during which the brain is more vulnerable to developing OCD. These studies will lead to a better understanding of how dysfunctional circuits lead to OCD symptoms, which is necessary to guide development of new treatments for this severe mental illness.